In recent years, more and more attention have been attracted to the process for converting oxygenates such as alcohols (methanol and ethanol), ethers (methyl ether and methyl ethyl ether) and esters (dimethyl carbonate and methyl formate) etc. to olefins (OTO), especially methanol to olefins (MTO).
U.S. Pat. No. 4,499,327 made a detailed study on the use of silicoaluminophosphate molecular sieve catalysts during conversion of oxygenates to light olefins and wherein it is believed that SAPO-34 is the most preferred catalyst for OTO process.
As well known in the art, SAPO-34 catalyst has high selectivity to light olefins and activity. However, in order to ensure high selectivity to light olefins, a required carbon deposition on SAPO-34 catalyst is necessary, therefore, the average carbon deposition on the regenerated catalyst in the regeneration zone has to be controlled at a required level so that the average carbon deposition on the catalyst in the reaction zone can be controlled accordingly. Thus, it is critical for MTO process to control the average carbon deposition on the catalyst in the reaction zone.
US 20060025646 relates to a method for controlling the carbon deposition on the catalyst in the reaction zone during OTO process, wherein a part of the deactivated catalyst is introduced to the regeneration zone to be regenerated by burning out the carbon deposition and the other part of the deactivated catalyst is recycled to the reaction zone for further reaction.
However, the disadvantage of the above-mentioned method is that a big difference is present between the carbon depositions of the deactivated catalyst and the regenerated catalyst entering into the reactor and neither catalyst with more carbon deposition nor catalyst with less carbon deposition is favorable to the selectivity to light olefins, so the light olefin yield would be influenced adversely.
The object of the present invention is further improving the control of the carbon deposition on the catalyst in the reaction zone during OTO process in the prior art, thus further increasing ethylene and/or propylene yield during OTO process.